Management of honey bee colonies may contribute to Varroa populations, study shows

From IPM in the South
By Rosemary Hallberg

Close proximity of honey bee colonies may contribute to Varroa population growth and virus transmission, according to an article recently published in Environmental Entomology. Varroa just detach from their current host and hitch a ride to another colony on a visiting foraging bee.

Varroa mites don’t reproduce very fast. A female mite will produce one to three offspring; infestations take several years to reach levels that would threaten the hive. However, in managed honey bee colonies, varroa populations increase rapidly, causing beekeepers to apply up to seven miticide applications per year.

To figure out why varroa populations were increasing at levels not typical with normal reproductive growth, a group of USDA Agricultural Research Service scientists studied 120 honey bee colonies in one commercial bee operation. The colonies were divided into two groups of 60, with one group treated with miticide in the fall and winter and the other treated in the spring, summer, fall and winter.

In their native countries in Asia, varroa does little damage to honey bee colonies because adult bees remove the mites from each other during hygienic rituals. In wild bee populations in the U.S., where hives are often quite some distance apart from each other, bees exhibit swarming behavior to start new colonies. When bees swarm and leave the nest, the old queen leaves with them, leaving a new virgin queen and no brood. This swarming behavior often results in the loss of about 35 percent of the varroa population.

When the research group simulated situations where bees did not swarm, mite populations increased substantially, and the colony died the next spring. Colonies that swarmed, on the other hand, survived while mite populations remained small for a five-year period.

In managed apiaries, beekeepers keep colonies closer together and add more wax comb to hives, reducing swarming. In addition, mites can infect colonies with several viruses, the most serious of which is deformed wing virus.

Deformed wing virus affects the brain of the honey bee, impairing vision and olfactory processing. In the end, the virus affects flight and homing performance. Colonies showed no sign of wing abnormalities before varroa began infesting hives. Hives seriously infested with varroa will usually collapse the next spring.

Because varroa depend on the brood and workers to survive, if the hive collapses, the mite colony dies as well.

In the fall, bees from stronger colonies often forage in hives where the colonies are weaker, robbing them of food resources. Other foragers may be bees that fly into the wrong colony. In either case, the researchers hypothesized, varroa may detach from some of the weakened bees and attach to the stronger bees, following them to a new colony where they can begin to spread.

The researchers observed two different weather conditions to study how varroa populations compared. One condition simulated “winter confinement” or hibernation, where bees did not rear brood or forage from late November to late January. The second condition simulated colonies where bees were active during the winter, where bees reared brood and foraged during the day.

The study revealed that mite numbers were higher in colonies that were active during the winter. Scientists advised that beekeepers should treat the hive in late winter or early spring to prevent colony collapse.

To keep colonies alive, the scientists recommend an integrated pest management system that combines resistance management, miticide rotation, the use of mite-resistant lines and changes in beekeeping practices, including spacing colonies apart to encourage or allow swarming. Other management practices include putting hives in cold storage or storing them indoors in the winter.

Source: DeGrandi-Hoffman, G., Ahumada, F. and Graham, H. (2017). Are dispersal mechanisms changing the host-parasite relationship and increasing the virulence of Varroa destructor (Mesostigmata: Varroidae) in managed honey bee (Hymenoptera: Apidae) colonies? Environmental Entomology, doi: 10.1093/ee/nvx077

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